This invention relates to an internal combustion engine control apparatus incorporating a thermal type intake air flow measuring instrument, and more particularly to a combination of a driver circuit of a thermal type intake air flow measuring instrument and a control circuit of an internal combustion engine control apparatus.
Among well-known intake air flow measuring instruments for internal combustion engines (hereafter briefly referred to as air flow sensors), there is an apparatus for detecting an air flow rate, which uses a heat-generating resistance element and an air temperature-compensating resistance element, both having temperature dependency and being installed in the intake air flow passage of the engine. A feedback circuit controls a current flowing through the heat-generating resistance element to keep the temperature difference between those two resistance elements at a constant value and detects a quantity of electricity corresponding to the current mentioned above so as to determine the air flow rate. The use of a microprocessor to control an internal combustion engine according to a current flowing through a heat-generating resistance element is well known, as this art is revealed in JP-A-61-137017. In the prior art apparatus, the microcomputer for control of the internal combustion engine was completely independent of the driver circuit of the air flow sensor. The microcomputer for control of the internal combustion engine controls ignition timing, injection quantity, etc. according to measured quantities, given in signals, such as a number of revolutions N of the internal combustion engine, an air-fuel ratio .lambda. from an O.sub.2 sensor, and an air quantity Qa from an air flow sensor. For an ordinary thermal type intake air flow sensor, a heat-generating resistance element and a resistance element for air temperature compensation are provided inside the suction pipe of the internal combustion engine, and a signal processing unit installed near those resistance elements converts signals to specified analog or digital form and supplies the signals to the microcomputer. And, for the sensor, there is provided a current control unit to control a current flowing through the heat-generating resistance element so that a difference in electrical resistance, that is to say, a temperature difference between the heat-generating resistance element and the resistance element for temperature compensation becomes a predetermined value.
Therefore, the conventional air flow detecting apparatus has to transmit detection signals to a microcomputer for control of an internal combustion engine one to two meters away. There is a possibility that noise enters on the signal line in the course of the detection signal transmission. In the area of hardware, the detection apparatus has to be in a large and complicated construction, so that the production cost is high.